Fluid-transfer pipelines are normally constructed using sections of metal tubes having a circular cross-section and laid end to end and connected in a fluid-tight manner.
It is known, in fact, that a thin circular metal wall can withstand appreciable internal pressures. Accordingly, for reasons of manufacturing economy and transport, pressurized pipelines are made of metal, at least on all occasions when external excess loads are not appreciable, e.g., in the case of pressure pipelines. However, when the lines are laid, a pipe incorporating a thin metal wall can become out of round because of its own weight, thereby hindering welding. This phenomenon then requires installation of stiffening elements which complicate construction, add weight to the pipeline, and, if these elements are left in place, form obstacles to fluid circulation.
In many cases, e.g., in sanitation or water-piping networks, the pipeline is placed on the bottom of a ditch and buried beneath a fill. The ditch is normally filled before pressurization of the pipe, which in this empty state, is thus subjected to the exterior forces generated by the fill and to potential over loads. After operational start-up, these external forces are counterbalanced by the internal pressure, which may vary, however, and can sometimes be absent. In all of these cases, the load pressing on the line can draw the pipe out of round and even crush it.
For this reason, buried pipelines are habitually made either of metal tubes possessing rather substantial thickness and incorporating exterior stiffening elements, or reinforced concrete tubes, which may or may not be pre-stressed.
Nevertheless, the transport and installation of these tubes requires limiting their dimensions and, consequently, the cross-section of flow, their diameter not exceeding, in practice, 2.5 meters, in order to remain within the highway gauge.
To solve this problem, proposals have already been advanced for constructing buried, large cross-section pipelines by utilizing assembled metal panels which normally have an undulated profile which enables them to better withstand external over loads and differential settling phenomena. However, these structures are not equipped to withstand an internal pressure, and there is a degree of risk that the pipeline will be crushed under the effect of external loads.
To build large cross-section pipelines capable of withstanding simultaneously an internal pressure and external excess loads, the same inventor has previously proposed, in Patent No. U.S. Application No. 5,061,121 or U.S. patnet Ser. No. 07/772,242, construction of a mixed-type tubular enclosure having a truncated cylindrical section and comprising, in cross-section, a lower part in the form of a massive slab made of reinforced concrete and an upper part shaped like an inward-curved arch formed by a thin metal wall whose two lateral ends are attached, respectively, to the concrete slab, each end being connected by a junction piece cemented to the upper face of the flat slab and being joined tangentially to the corresponding lateral end of the arch.
This mixed construction allows optimal use of materials.
In fact, when the pipeline is pressurized, the metal arch is subjected solely to traction stresses and can be made from rather thin thin-walled elements which are easily maneuvered and welded together so as to ensure fluid-tightness under pressure.
Furthermore, because of the truncated cross-section at its base, the upper metal part effectively withstands external forces, even when the line is empty, by virtue of an arch effect. The flat base, which acts as a stiffening element, is subjected to flective stresses, yet can withstand them under favorable conditions, because this base is formed from a concrete slab whose thickness and iron framework can be determined as a function of the stresses it supports. Moreover, this flat base gives the pipeline an effective foundation surface, which makes it possible to spread out the stresses applied on the ground and to withstand differential settling phenomena.
The slab may advantageously be poured at the site as the work progresses, or it may be formed from prefabricated elements laid down in succession.
In U.S. Pat. No. 5,061,121 and U.S. patent application No. 07/772,242, previously cited, the metal arch is connected to the slab along its two lateral edges by means of shaped sections comprising an upper part attaching tangentially to the arch, and a lower part cemented to the upper face of the slab in such a way as to make it possible to withstand tearing stresses and, at the same time, provide fluid-tightness.
This attachment method is well adapted to the case in which the slab is made of prefabricated elements. However, it can complicate construction when the slab is poured on site. Furthermore, when strong internal pressures are generated, it may be feared that the concrete will ultimately crack as a result of the stresses generated.